Barium strontium titanate, also referred to herein as BSTO, has been known to be used for its high dielectric constant (approximate range from 200 to 6,000) in various antenna applications. This is set forth by Richard W. Babbitt et al. in their publication entitled, "Planar Microwave Electro-Optic Phase Shifters," Microwave Journal, Volume 35 (6), June 1992. This publication concluded that a need existed for materials having more desirable electronic properties.
To address this need, BSTO has been combined with various oxide additives such as MgO, Al.sub.2 O.sub.3, ZnO, ZrO.sub.2, and magnesium based compounds MgZrO.sub.3, MgAl.sub.2 O.sub.4, and MgTiO.sub.3. See, for example, U.S. Pat. Nos. 5,312,790; 5,427,988; 5,486,491; 5,635,433; and, 5,635,434 hereby incorporated by reference herein.
Although various types of these ferroelectric composite materials are known, prior art methods of making these materials are not conducive to the use of these types of materials in ferroelectric phase shifter antenna applications at high frequencies, i.e., 30 GHz and upwards. In these applications, it is desirable to have the ferroelectric material in thin films having a thickness of 1 micron (.mu.m) or less. It is difficult to polish ceramics of these thicknesses and at the same time handle them properly for insertion into a given device.
As such, there exists a need for the fabrication of ferroelectric composite materials having improved electronic properties and also having the thin film structure ideal for use, for example, in multilayer capacitors, capacitor-varistors, semiconductor devices such as dynamic and non-volatile random access memory (DRAM) computer memory cells, or for use in phased array antenna systems. There is also a need in the multilayer ceramic capacitor market for the ability to fabricate these materials in forms that may assist in increasing the component density of the circuit. Thus, there is a growing need for miniaturization, large capacitance and low cost in this market.
A variety of techniques have been used for the deposition of ferroelectric thin films. In general, the thin film deposition can be divided into two major categories: (1) physical vapor deposition (PVD); and (2) chemical processes. Among the PVD techniques, the most common methods used for the deposition of ferroelectric thin films are electron beam evaporation, rf diode sputtering, rf magnetron sputtering, dc magnetron sputtering, ion beam sputtering, molecular beam epitaxy (MBE), and laser ablation. The chemical processes can be further divided into two groups, i.e., metalorganic chemical vapor deposition (MOCVD), and wet chemical processes including sol-gel process and metalorganic decomposition (MOD).
The advantages of the wet chemical process are: (1) molecular homogeneity; (2) relatively high deposition rate throughput; (3) good composition control; (4) easy introduction of dopants; and (5) low capital cost since deposition can be done in ambient condition so that no vacuum processing is needed. The major problems due to this wet process are: (1) film cracking during post annealing process; and (2) possible contamination which results in difficulty in incorporating this technique into semiconductor processing. However, because it provides a fast and easy way to produce complex oxide thin films, this wet chemical process has a very important role in the investigation of the interrelationship among the processing, the microstructure, and the property of the films. See, for example, U.S. Pat. No. 5,625,587.
The deposition of BSTO/MgO composite in thin film form has been demonstrated by the pulsed laser deposition (PLD) method in U.S. Pat. No. 5,766,697. However, it has not been suggested in the prior art that the MOD process may be used to produce thin films of BSTO/Oxide composites in two-phase and having improved electronic properties. Moreover, particulate-free large area deposition is generally not possible, or is at least difficult, by the pulsed laser deposition technique and the uniformity of the thin films deposited by pulsed laser deposition decreases with increasing surface area. The present invention presents for the first time a method of making thin films of BSTO/MgO, wherein the electrical properties of the thin films are greatly changed by the addition of magnesium and the addition of magnesium also results in a secondary phase in the film.